The present invention relates to a structure and method of manufacture of a ceramic case for implantation in a living body, and more particularly to a ceramic case that has enhanced strength due to the use of a potting material or encapsulant to support the ceramic walls of the ceramic case.
Electronics packages that are implanted in living bodies must be housed in packages constructed of biocompatible materials. These packages must protect the electronic circuitry located within them from body fluids and ions so that the electronic circuitry can survive for extended periods of time without any significant changes in performance.
Today, the most commonly used metals for implantable packages are titanium, stainless steel and cobalt-chromium alloys. These metals are biocompatible and corrosion resistant. Normally, the package construction consists of parts that are welded together to insure hermeticity. However, where there is a need to inductively couple an alternating electromagnetic field to an internal pickup coil, the metal package becomes a hindrance. Such is the case for an implanted stimulator that is powered from an external source. Specifically, transmission of power is substantially reduced by eddy currents generated in the metal package due to the alternating electromagnetic field. To solve that problem, receiving coils may be placed on the outside of the metal package, increasing the size and complexity of the implanted device.
Glass and ceramic material represent viable materials for an implantable medical device package because they are transparent to alternating electromagnetic fields. Receiving coils can be placed inside a hermetic zone of a ceramic or glass package, creating an overall smaller and simpler implant device and reducing the possibility of coil failure due to saline leakage. Advantageously, glasses and ceramics are inert and highly insoluble, which are favorable characteristics for long term implant materials.
Unfortunately, ceramics and glasses are inelastic and fragile when subjected to tensile stresses such as the stresses generated under mechanical shock or impact. Additionally, they are subject to fracture not only from mechanical shock but also from differential thermal expansion if even a moderate temperature gradient exists thereacross. Therefore, welding is not a practical method of sealing glass or ceramic materials. Instead, if a ceramic package is used, virtually the entire package and its contents must be raised to the high melting temperature of the ceramic or metal braze that is used to effect a sealing of the ceramic package. Such high-temperature sealing methods are unsatisfactory.
One type of hermetically sealed ceramic and metal package is shown in U.S. Pat. No. 4,991,582, issued to Byers et al. and incorporated herein by reference. A ceramic case and a metal band are hermetically sealed together, each being characterized by similar coefficients of linear thermal expansion. The electronic circuitry is then loaded inside the package, and final package closure is effected by welding a metal header plate to the metal band. Disadvantageously, the electronic circuitry is unsupported and is thus susceptible to damage from an impact force.
In view of the above, it is evident that what is needed is a ceramic or other package that has improved mechanical impact resistance and can protect the electronic circuitry carried inside of the package.
The present invention advantageously addresses the needs above as well as other needs by providing an implantable medical device, e.g., an implantable cochlear stimulator (ICS), having a ceramic or similar case that exhibits enhanced case strength.
The material, e.g., ceramic, from which the case of the present invention is made is inert to body fluids. The majority of the case is made of biocompatible ceramic material having an open end. Electronic circuitry and a selected potting material are placed inside of the case through the open end. A closed metal band or ring is then, e.g., welded, to the open end of the ceramic case. The metal band is made from a biocompatible material that has the same coefficient of thermal expansion (CTE) as the ceramic material. The attachment of the metal band to the ceramic case may be effectuated in any suitable manner, e.g., it can either be a butt joint or one of the components may have a fixturing ring or step for self jigging. The attachment of the metal band also provides a hermetic seal. The preferred method of attachment is brazing the metal band to the ceramic case using a metal or metal alloy braze. Advantageously, such brazing, while performed at a high temperature, may be done without the electronic circuitry being present.
An electronic circuit assembly (hereafter also referred to herein as xe2x80x9celectronic circuitryxe2x80x9d, xe2x80x9ccircuitryxe2x80x9d, or xe2x80x9celectronicsxe2x80x9d) is sized to fit inside of the ceramic case. Prior to the insertion of the electronic circuit assembly into the ceramic case, the ceramic case is filled with a non-cured potting material that is in a fluid state. When cured, the potting material thereafter advantageously supports the fragile walls of the ceramic or other case and prevents such walls from collapsing under an impact or load. Moreover, the potting material also supports the electronic circuitry. The uncured potting material is initially soft, having the constituency of a high viscosity fluid (e.g., much like ketchup) to allow the electronic circuitry to be inserted into the ceramic case, displacing some of the potting material and allowing the potting material to fill the voids between the electronic circuitry and the inside walls of the ceramic case. Once the electronic circuit assembly is inside the ceramic case, the potting material cures, making it rigid. The header is then welded to the metal band, sealing the package. The header has a plurality of electrical connectors (electrical feed-through terminals) passing through it for connecting electrical leads of the electronic components inside of the package. Once thus assembled, the implantable electronic device within its ceramic (or other) case is ready to be implanted in a living body.
An added benefit provided by the invention is that the potting material acts as a trap or collector for any moisture that might enter the hermetic package, thereby preventing condensation of moisture on the microcircuits and other delicate components that form part of the electronic circuitry. Such moisture trap or collector thus extends the useful life of the device.